Electrically programmable fuses have been used extensively in the design and manufacture of Very Large Scale Integrated Circuits (VLSI).
Fuses are oftentimes used for redundancy purposes, wherein certain defective regions within an integrated circuit (IC) chip or an IC assembly can be electrically disconnected and replaced by a functional region. This technique of programming fuses has the advantage of increasing yield and adding flexibility to the circuit designer, providing ease of "repair", e.g., engineering changes (EC), in a manner well known to those skilled in the art.
Typically, a fuse may be programmed by a laser in an operation known as "laser zapping", wherein the fuse is physically heated and ablated. Other programming techniques include passing an electrical current through the fuse link in the form of a pulse such that the fuse resistance causes joule heating, which thermally melts and electrically isolates electrical interconnections from one another. In both instance, energy is applied directly to the fuse during the programming phase.
By way of example, in UK Patent No. 2,237,446 A to Machida et al., a process is described wherein an opening is formed over a fuse having two terminals which connect the fuse to external wiring and to a programming source. A fuse according to the teachings of Machida et al.is shown in FIGS. 1a and 1b.
Alternatively, in a second typical fuse layout, energy can be applied indirectly to the fuse, as shown in FIGS. 2a and 2b. This layout is described in U.S. Pat. No. 5,084,691 to Lester et al., wherein a controllable fuse having a heating element (or elements) 12 is shown to provide sufficient energy to melt a low temperature solder alloy 11. The fuse is disconnected by the source of energy placed in close vicinity to the fuse which raises the melting temperature to a level sufficiently high to ensure discontinuity.
Inherent to fuses of the type described by Machida et al. and Lester et al. as well as by similar structures known in the art, are two terminals through which current passes during the programming phase of the fuse. Common to all fuses is a combination of joule heat and electrical current through two terminals which actually causes the fuse to become discontinuous.
While the teachings of fuse formation and integration are well understood by those skilled in the art, fuses of these types suffer from serious drawbacks.
First, when an IC chip is powered, transient currents and voltage "spikes" are always present in the wiring which cause the power in the lines to fluctuate. This situation is increasingly important as chip integration increases and the circuit dimensions are scaled down to submicron range. Second, oftentimes, electrical noise in the environment can cause voltage fluctuations which can exceed the specifications of the IC chip design. If either of these fluctuations or noise "spikes" occur, it is conceivable that a fuse of the type previously described may reach a point above the threshold required for accidental programming, thereby inadvertently "blowing" the fuse.